High intensity neutron source

ABSTRACT

Disclosed is a neutron generator in which a gas (such as a heavy isotope of hydrogen) or a mixture of gases, is ionized by any convenient means such as exposure to a magnetically stabilized arc. The ions are directed to an accelerator and the resulting high velocity ion stream is caused to impinge on a target. The interaction of the ion beam with the target material soon builds up a high density of gas in the target surface which, in turn, interacts with the incoming beam to produce an intense isotropic neutron output from the fusion of the isotopes.

United States Patent [191 Hilton et al.

HIGH INTENSITY NEUTRON SOURCE Inventors: John L. Hilton, Walnut Creek;

Gordon W. Hamilton, Livermore, both of Calif.

Assignee: The United States of America as represented by the UnitedStates Atomic Energy Commis sion Filed: Apr. 22, 1970 Appl. No.: 30,724

US. Cl ..250/501, 313/61 S Int. Cl G2lg 3/04 Field of Search 250/845;313/61 S References Cited UNITED STATES PATENTS [451 July 17,1973

Primary Examiner-James W. Lawrence Assistant Examiner-Davis L. WillisAtt0rneyRoland A. Anderson [5 7] ABSTRACT 2 Claims, 1 Drawing Figure3/]964 Reifenschweiler 250/845 X g 6 2 m PUMPING 3 SYSTEM 32 3 l 52 asas wk 64 78 a s m j S 2 J O Patented July 17, 1973 3,746,859

PU MPI N G SYSTEM (/OH/V L. 7 TO/VA 60200 M/ v /A'M/L 70/1/ INVENTORSmam ATTORNEYS 1 HIGH INTENSITY NEUTRON SOURCE BACKGROUND OF THEINVENTION Intense beams of high energy particles such as neutrons haveimportant applications in analysis of materials, therapy (especiallycancer therapy), nuclear reactor development, nondestructive testing,etc. and considerable attention has been directed to extending theuseful life of the beam neutron output by water cooling the generatingtarget, providing a good vacuum, reducing the sputtering of the targetmaterial, and in deuteriumtritium neutron producing systems, bypreventing deuterium dilution of the tritium-loaded target.

Satisfactory solutions to most of the above problems have minutes. foundin present-day technologies of cooling, high vacuum techniques, mixedisotope beams and the physical chemistry of materials. However, becauseof target deterioration, there does not yet exist a high neutron yielddevice which permits a small source size with long life. Typically,assuming a 6 cm stationary target, a neutron output of 10neutrons/second requires the order of 10 ma at 300 Kv, if a puredeuterium beam is accelerated into a previously loaded tritiated target.This has a deuterium dilution half life of only 36 If the dilution isovercome by using a mixed beam (requiring twice the current) is used,the necessary thin and delicate target surface plating is sputtered oreroded away in a comparatively short time.

BRIEF SUMMARY OF THE INVENTION The present invention provides astructure capable of generating a high intensity neutron yield, forwhich structure the target life is considerably extended as a result ofthe incorporation, on a properly cooled surface, of a high beam currentdensity deuterium-tritium beam. This is achieved by using a mixture ofboth gases in the ion source and target of deuterons and tritons (i.e.,a mixed beam); this high beam current density ion beam is accelerated atan energy of the order of 100 Rev or higher onto a mixed gas targetformed near the surface of a relatively thick stationary metallictarget. The target gas concentration is maintained in dynamicequilibrium by continuous replenishment by the beam and can achieve auseful life limited only by the sputtering of the thick target. Thus, inthis generator, it is not necessary to change the target frequently and,consequently, higher neutron yields per source area, economy andoperational convenience are achieved.

BRIEF DESCRIPTION OF THE DRAWING The FIGURE shows the neutron source ofthe present invention in longitudinal cross-section.

DESCRIPTION OF PREFERRED EMBODIMENT Before discussing the shownpreferred embodiment which is just one of the several possibleconfigurations, it may be appropriate to outline some of theconsiderations which the present inventors had in mind prior to andduring its development.

When a properly cooled simple Class A metallic target (e.g. Al, Cu, Mo,Ni, etc.) is struck by energetic deuterium and tritium ions, theconcentration of hydrogen ions near the target surface, due toimplantation by the beam, rapidly exceeds that expected from normalhydrogen solubility in the metal. As the concentration builds up, theneutron production level also rapidly increases. If a uniform currentdensity in the beam is assumed, a one dimensional analysis (infiniteextent in the other two dimensions) indicates that the hydrogen willdiffuse out of the target, at a rate proportional to the concentration,until the rate of diffusion is equal to the rate of implantation; thispresumes that all the diffusion is to the surface of beam impacts, whichis a valid presumption since the target thickness is large compared tothe l to 3 micron penetration depth of the beam. This limiting steadystate condition may be represented by where i is the beam currentdensity e is the electronic charge D is the diffusion coefficient dn/dxis the gradient in the number density of the hydrogen.

The neutron yield per square centimeter of target area, which isproportional to the product of the beam current and the density of thehydrogen in the target, would therefore vary as the square of thecurrent density if all other factors are constant. However, thediffusion coefficient is a strongly increasing function of temperature,given theoretically by lD(T) D T" exp (b,,/T). An increase in beamcurrent density will generally result in an increased target temperatureand a related increase in the diffusion coefficient; Thus because ofcooling limitations, the full i dependence of the neutron yield cannotbe realized.

The neutron yield using a simple metallic target is therefore expectedto follow the proportionality:

Ya Ai /D(T) where y is the neutron yield and A is the area of the targetirradiated by the beam.

For mono-atomic single energiedl beams, the concentration of hydrogen inthe target increases linearly with depth to the point where the ionsstop. This penetration depth for a titanium target, is approximately 2.2p. for KeV D and -2.8 p. for 170 Kev T and D, while the most probablefusion depth is between I to 1.5 p. for 170 KeV T and D. Therefore, theneutron yield should continue to increase with beam energy, but with adecreasing slope, as the energy is increased. For this type of target,the largest yields will result from the highest practical beam energy,the largest current density consistent with heat transfer requirementsfor the target, the largest area (consistent with any limits on sourcesize that may be imposed) which implies the largest total beam current,a low diffusion coefficient of the target material at the operatingtemperature, and the lowest possible target temperature.

Aluminum and copper, have low diffusion coefficients for hydrogen and arelatively high thermal conductivity; they are attractive targetmaterials, but because of its light atomic mass aluminum is expected tosputter at a higher rate than would a heavier target material such ascopper.

The target temperature will be proportional to N, where V is theaccelerating voltage of the ion source. Since heat transfer from thetarget :is expected to be the limiting factor in neutron yieldoptimization, this will limit the values of i and V that can be used forany given size or target.

Metal targets made of Class B metals (e.g. Ti, Er, Zr), if keptrelatively cool, can initially contain a large quantity of hydrogen,apparently as a pseudo-hydride. Irradiation of such a pre-loaded targetby a D and/or T beam will initially produce a high neutron yield,proportional to the total beam current. Again the yield will be higherfor higher beam ion energies and the neutron yield will remain highuntil the hydride has been sputtered off, or the hydrogen has partiallydiffused out, or, in the case of a pure D beam impinging on a puretritiated target, until the tritium in the target has been diluted bydeuterium. At this point, the yield from the hydrogen isotopes initiallycontained in the target will decrease; however, the yield from hydrogenimplanted by the beam will remain. Although the diffusion coefficientfor hydrogen in the Class B metals is generally very large, the hydrideformation'apparently traps the hydrogen in the target structure, givinghigh hydrogen concentrations if the target temperature is not too high.Thus, the hydrogenconcentration in a Class B metal target is notexpected to be a function of the diffusion equations alone; theconcentration may be augmented by hydride formation.

At higher beam current densities, the concentration of hydrogen trappedin the hydride becomes less important. For example, at a current densityof 1.3 mA/cm and a total current of mA, the yield from a well-cooledtarget with implanted hydrogen diffusing out of it is about half that ofthe optimum yield from the hydride. At higher beam densities, if thetemperature is held constant, the difference is less important.

With regard to target erosion rates, measurements made with a high beamcurrent at a controlled temperature suggest a large difference betweenvapordeposited titanium Ti on a copper substrate and cold rolledtitanium sheet stock; the sheet target withstands three times theoperating period that completely destroys the plating under similartarget cooling and beam intensities. Therefore, the use of nonplatedsheet metal targets, on which are built up the hydrogen isotope mixtureby a drive in mechanism, is preferred. These targets can have athickness limited only by heat transfer and will not be eroded asquickly.

The mixed beam approach eliminates the regular tritium depletion problemalong with another problem that is evident as target beam currents areextended from the presently available 0.1 mA/cm to to 50 mA/cm. Thisproblem arises as the pure deuteron beam loads the target surface withthe beam atoms. At a beam current of 0.1 mA/cm, this effect isrelatively small, but it becomes significant at the high currentdensity. For a total 75-150 mA beam on a target spot of 3 to 5 cm, nearsaturation of the target occurs in a hundred seconds or so. While thisis an advantage for the drive-in-target on a mixed beam accelerator, itwill limit the pure deuteron beam tritiated foil system to a target lifeof no longer than a few minutes (or perhaps to an hour or so if a largerotating target is used).

During operation, hydrogen concentration in the target will continue torise until the hydrogen (both D and T) leaks back out of the targetsurface as fast as it is injected by the beam. At ambient temperature,this concentration will occur with an e folding time of approximatelyone hundred seconds. Therefore the incoming pure deuteron beam willquickly dilute the needed tritium and both isotopes will outgas from thetarget, giving the appearance of an enhanced tritium depletion in amatter of a few minutes.

The mixed beam accelerator eliminates not only target dilution but alsoanother potential problem of the pure deuteron beam. Within minutes, apure D accelerator significantly reduces the target tritium byimplanting deuterium. For a mixed beam system, where there is an evenratio in the feed gas, the target isotope ratio will, of course, alwaysremain at nearly 50 percent. With too high a D to T ratio the totalyield will drop and the unwanted D-D neutrons may become an increasingfraction, producing a serious problem in both diagnostic and therapyapplications of a generator. As long as the D to T ratio in the targetremains close to 50 percent, it does not produce an objectionablequantity of D-D neutrons, the ratio of the D-D to the D-T fusioncross-section reduces unwanted low energy (i.e., 2.45 MeV) D-D neutronsto about two-thirds of 1 percent of the 14 MeV neutrons (for anaccelerator energy of 150-170 KeV).

A useful system could involve a total beam current of -l50 mA at -170KeV energy with an ion beam current density of 10-30 mA/cm the totalpower deposited in the target is therefore 13 to 25 Kw over the beamdiameter (4 8 Kw/cm perpendicular to the beam for a 2 cm spot diameter).

The useful beam power density is limited by the ability of the target todissipate this beam power and maintain a fairly low target surfacetemperature. An elementary heat transfer computation for a thinnon-moving target cooled at the rear surface, shows that the limitingbeam power density is proportional to KAT f/p where K is the thermalconductivity of the target material,

AT is the temperature difference between the target surfaces f is theallowable tensile stress p is the collant pressure The target is assumedto be as thin as is structurally possible, and therefore the thicknessis proportional to f/p- The targets surface temperature must be keptwell below its melting point to avoid excessive outgassing of theadsorbed D-T. In selection of a target material for a drive in target,it is desirable to maximize hydrogen content, which is a function of thediffusion rate, as well as K, T and f. Among common target materials,aluminum, copper, nickel and molybdenum are good choices with respect tothese figures of merit.

It is practical to build targets at a simple or compound slope to thebeam. A fixed target shaped to intercept the beam on a compound slopedsurface, such as the surface of a cone could be used. The neutron yieldcan be observed from the downstream side, since the absorption of thedesirable 14 MeV neutrons as they pass through the target will berelatively small; the downstream diameter of the resulting neutronsource spot" will still be small, while the heat transfer requirementwill be reduced. This will, of course, detrimentally reduce theeffective beam current density and shorten the diffusion path length outof the target surface.

It is, of course, necessary to maintain a. stable target temperature byremoving the beam energy from the target. It is possible to utilize theimproved thermal transfer of a rotating target. However, this type ofdevelopment indicates a comparatively high cost, and significantreduction of the target loading. Therefore, a non-rotating target isconsidered preferable. A beam current density of at least 23 Kwlcm orapproximately 140 mA/cm, can be tolerated in a water cooled nonrotatingcone target, if melt-down heat transfer were the only limitation oncurrent density. The optimum shape for a fixed target will probably be aright cone with the base diameter about one half the depth. For thisshape, the surface area capable of cooling is 4.12 times the surfacearea of a flat plate perpendicular to the beam. Further, the coolantflow rate is highest at the center, where the highest beam density andheat flux are encountered.

A cone-shaped copper target with a depth-todiameter ratio of 2 manifestsa surface temperature of 200-225 C for a 100 mA beam at 175Kv, and aspot diameter of approximately 2 centimeters of the water coolant flowrate is 30-35 ft/sec across the back side of the target plate. Also itis possible to cool a thin bonded titanium sheet on a back-up plate ofcopper or other Class A metal not subject to hydrogen embrittlement.This could possibly maintain a surface temperature below 300 C at thesepower densities, although Ti has a relatively poor thermal conductivity.However, best yields will be obtained from target temperatures lowerthan can be obtained from a water coolant, at these power densities.This well-cooled target in combination with the high current densitybeam will provide startlingly higher yields.

Turning now to the preferred embodiment shown in the FIGURE, the neutronsource may be regarded as comprising ion source 10, accelerator 112 andtarget 14.

Ion source has been described in the publication Plasma Physics, Volume10, pages 687 to 697, Multimomentum 650 ma Ion Source" by G. W.Hamilton, J. L. Hilton and J. S. Luce as a further development of a ionsource considered in the U. S. Pat. application, Ser. No. 712,197, filedMar. 11, 1968. Briefly, a mixture of elemental gases, typicallydeuterium and tritium is supplied from a reservoir (not shown) at inletto the area separating cathode 22 and anode 24 between which powersupply 26 provides an arc discharge, thereby resulting in an ion plasmaconsisting of the nine Species 2( 3( A z T(3), T (6) and T (9), wherethe numbers in parenthesis are the ion mass numbers. A pair ofelectromagnets 30, 32 constrain the resulting ion plasma in area anddensity; electromagnet 32 is operated with a polarity opposing that ofelectromagnet 30 so that the resulting flux line can be controlled indensity and area. An ion beam is extracted and electrostatically focusedby a three level arrangement consisting of reflector electrode 34,extractor electrode 36 and decelerating electrode 38, fed from powersupplies 40, 42. Typical operating voltages are, between electrodes 34and 36 +10 to +20 Kv and between electrodes 36 and 38 -2 to 4 Kv. Thebeam is fed through apertures 44 in electrodes 34, 36, 38, and throughhalo baffle 46 into vacuum vessel 48. [on source 10 is cooled asappropriate through coolant fittings 50, 52. Halo baffle 46 operates toprotect accelerator 12 from the diffuse, nonfocussed halo surroundingthe ion beam as well as to protect ion source 10 from high energy,backstreaming electrons.

The active elements in accelerator 12, which is entered by the beamthrough entry 56in case 58, are electromagnet 54, of about six Kgausecapacity, which collimates the beam, and case 58 is energized by voltageon the order of l l0l Kv to accelerate the beam to ward target 14.Electromagnet 54, as shown, is oriented coaxially. The voltage issupplied to case 58 through insulator 60, and insulator 60 also servesas partial support for accelerator 12. Flow of coolant is providedthrough fittings 64, 66. With regard to structural materials, case 58 isof mild steel to provide a magnetic flux return path, entry 56 is ofcopper brazed to case 58 and copper separator 57 such that the electricfield abode entry 56 and the fringing magnetic: lines are separated.

The beam emitted from accelerator 12 impinges upon target 14, therebyreleasing neutrons isotropically by the fusion reaction.

Target 141 comprises sheet 78 of a metal such as copper, aluminum,titanium, nickel or molybdenium, the rear surface of which is cooled bycoolant entering through fitting 76. The beam of ions (the hydrogenisotope mixture) impinges on the front surface of sheet 78 and builds upthereon a film of mixed hydrogen isotopes which, as it diffuses out andgenerates neutrons, is continuously replenished by the beam. With regardto target cooling, water may be used to give fair yields and life.However, results are surprisingly good if the cooling system is capableof very high power density at relatively low temperatures; an admirableexample has been found to be a non-boiling liquid metal heat transferloop such as the eutectic NaK or mercury to maintain a targettemperature at the range 75 to C. Other high heat flux systems are justas valuable if the front surface of sheet 78 can be maintained at thelow temperature while being heated by the high current density beam.

In the present system, the accelerator and ion source operate in arelatively high vacuum (3 X 10 torr), which reduces backgroundionization, backstreaming electrons and x-ray radiation, and the feedgas (T and D) is continuously recirculated through the system by afail-safe closed loop vacuum system, thereby minimizing danger due tooperating with several curies of tritium in a free gaseous form. Thus,gas is emitted from vessel 48 at fitting 47, piped to pumping system 62for compression and transfer back to inlet 20. Obviously, an additionalstorage inventory of the gas may be sealed in this recirculation loopand used as needed.

What is claimed is:

1. A neutron generator, comprising:

a source of ions;

beam-forming means for the ions from said source,

the beam including a plurality of isotopic ions;

an accelerator for the beam from said beam-forming means;

a target to receive the beam from said accelerator having a surfacecapable of occluding some of the beam atoms so as to emit neutrons as aresult of impact by the beam and capable of being replenished by theions of the beam, said target comprising a homogeneous metallic plate;

a vacuum vessel in which said accelerator and said target are enclosed,and

means to cool said target at its back side surface, said cooling meanscomprising a flow of liquid metal capable of maintaining said target ata temperature in the range of 75 to 200 C.

2. A neutron generator, comprising:

a source of ions;

beam-forming means for the ions from said source,

the beam including a plurality of isotopic ions;

an accelerator for the beam from said beam-forming by the ions of thebeam, and

means; a pumping system connected between said target and a target toreceive the beam from said accelerator said ion source operative toreceive gas in the vicinhaving a surface capable of occluding some ofthe ity of said target, compress the gas and transmit the beam atoms soas to emit neutrons as a result of imcompressed gas to said ion source.pact by the beam and capable of being replenished

2. A neutron generator, comprising: a source of ions; beam-forming means for the ions from said source, the beam including a plurality of isotopic ions; an accelerator for the beam from said beam-forming means; a target to receive the beam from said accelerator having a surface capable of occluding some of the beam atoms so as to emit neutrons as a result of impact by the beam and capable of being replenished by the ions of the beam, and a pumping system connected between said target and said ion source operative to receive gas in the vicinity of said target, compress the gas and transmit the compressed gas to said ion source. 